This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This study aims to determine the structure of the membrane ion channel, KcsA, in open and closed states. Terbium labeled KcsA mutants are incorporated into unilamellar lipid vesicles and exposed to X-rays at energies near the Tb+3 L3 edge. This solution X-ray scattering experiment takes advantage of an anomalous signal arising from cite specific Tb+3 labels.